This invention relates to optical measurements, and particularly to the measurement of the polarization mode dispersion of optical devices, particularly optical fibers used in telecommunication networks. Although the polarization mode dispersion measurement can be made at a single light wavelength, by making the measurements at plural wavelengths, other optical parameters, e.g., chromatic dispersion, can be determined.
Polarization mode dispersion (PMD) is a distortion mechanism (like chromatic dispersion) that causes optical devices, such as single-mode fibers, optical switches and optical isolators, to distort transmitted light signals. The relative severity of PMD (which is a function of the wavelength of the transmitted light) has increased as techniques for dealing with chromatic dispersion have improved, transmission distances have increased, and bit rates have increased. Negative effects of PMD appear as random signal fading, increased composite second order distortion and increased error rates.
PMD is due to differential group delay caused by geometrical irregularities and other sources of birefringence in the transmission path of the optical device. For example, a single-mode fiber (SMF) is ideally a homogeneous medium supporting only one mode. In practice, it supports two propagation modes with orthogonal polarizations. When a lightwave source transmits a pulse into a SMF cable, the pulse energy is resolved onto the principal states of polarization of the fiber. The two groups of pulse energy propagate at different velocities and arrive at different times causing pulse broadening and signal distortion.
Determining the PMD of in-place (installed) optical fibers is useful for determining the capacity of the fibers for transmitting new telecommunication services of ever increasing bandwidth requirements and for the design and control of PMD compensators. Also, because the PMD of any given fiber can change rapidly with time (owing to random asymmetrical stresses along the length of the fiber), repeated PMD measurements of the fiber can be used in a feedback system for compensating for variable PMD distortions.
The PMD of a fiber is commonly characterized by two specific orthogonal states of polarization called the principal states of polarization (PSPs) and the differential group delay (DGD) between them. This can be described at an optical angular frequency, xcfx89, by the 3-component Stokes vector, {right arrow over (xcexa9)}=xcex94xcfx84{right arrow over (q)}, where {right arrow over (q)} is a unit Stokes vector pointing in the direction of the faster PSP, and the magnitude, xcex94xcfx84, is the DGD. Typical DGD values encountered in transmission systems range between 1 (picosecond) ps and 100 ps.
Known methods for determining PMD vectors include the Jones Matrix Eigenanalysis (JME) technique and the Mxc3xcller Matrix Method (MMM). Each of these techniques uses a tunable, continuous-wave laser and a polarimeter to measure the output polarization states for two (or three) different input polarization launches at two optical frequencies. The PMD vector is then calculated for the midpoint frequency. In addition to determining the output PMD vector, the Mxc3xcller Matrix Method determines the rotation matrix of the fiber at each frequency and thus the input PMD vector can be calculated.
Shortcomings of these techniques are that they are somewhat difficult to implement, particularly in a feedback system, because they require frequency differentiation of measured data at plural optical frequencies. As described hereinafter, an advantage of the method according to the present invention is that it requires measurements of data at only one optical frequency. Also, while, as hereinafter described, measurements can be made at plural optical frequencies, separate, single frequency measurements are made that do not involve frequency differentiation.
A further advantage of the invention is that it can be used to accurately measure chromatic dispersion of an optical device in the presence of PMD. Other known methods for measuring chromatic dispersion tend to give inaccurate results if the devices have PMD.
At the input of an optical device, typically an optical fiber link within an optical fiber telecommunications network, four different light signals, all of the same optical frequency, but having different states of polarization, are transmitted along the fiber and the mean signal delay of each of the light signals is measured. By calculation (described hereinafter), the four mean signal delay measurements yield the desired PMD vector of the optical device. Additionally, by repeating the mean signal delay measurement at multiple optical frequencies (i.e., at a different optical frequency for each set of time delay measurements) determination can be made of the first and higher-order intrinsic (polarization-averaged) chromatic dispersion of the device being measured, as well as the higher-order PMD of the device.